This subject matter is generally directed to a method and a mask inspection microscope for characterizing a feature on a mask.
In the fabrication of semiconductor components, numerous measurement methods are used to monitor the results of the individual steps in the lithographic process.
To verify a process for generating features on wafers, it is advantageous to characterize the smallest features that can still be formed on a wafer. These are referred to as the critical dimension, abbreviated CD. The critical dimension of a feature usually refers to the line width of a feature composed of alternating lines and spaces. In the lithographic process, this feature is first formed on a mask (photomask, reticle), for example as chromium on quartz glass. There are also other known masks, for example phase shift masks (PSM) or reflective masks, that are used particularly with short-wavelength illuminating radiation in the EUV range. By exposure in a scanner, this feature is imaged onto a wafer coated with resist. The desired feature is produced on the wafer by subsequent developing and etching.
The characterization of a feature, particularly the CD or line width of a feature, can be performed both on the mask and on the wafer. Although on-wafer measurement yields very meaningful data, since the product is characterized at the end of the process chain, it is very involved, since the entire wafer exposure process has to be performed for the test.
If the characterization of a feature to be fabricated is performed on the mask itself, errors caused by the behavior of the mask during imaging and by other method steps are not incorporated into the measurement. Errors on the mask are usually intensified by the scanner during imaging. Another problem is that the mask features are known and are optimized by resolution enhancement technology (RET), and thus do not completely match the features that are to be imaged. This makes it difficult, for example, to measure the CD directly on the mask.
Both on mask and on wafer, CD measurement is performed by, for example, scanning electron microscopy (CD-SEM, critical dimension scanning electron microscopy).
Another way to measure the CD is to analyze the aerial images of masks with a mask inspection microscope. In this method, the aerial image shows most of the features that are also being projected onto the wafer.
Another way to characterize masks, such as by measuring the CD of masks and wafers, is afforded by non-imaging optical methods. In the case of transmissive masks, one notable method is to measure the transmission of structured regions of a mask, as disclosed, for example, in International Laid-Open Patent Application WO2009083606A1.
Non-imaging optical methods yield measurement values that are correlated with quality-related properties of the mask, for example the line width or CD of a mask. Masks can be characterized by measuring these values alone. By calibrating the method, it even becomes possible to determine absolute values.